Feed-back circuit



April 3, W511 T. A. RHCH ET AL 2,547,662

FEED BACK CIRCU IT Filed April 26, 1949 Amplifier 1 J Feed back (L) Theodore ARich John EBigelow,

The i r Att orney.

Patented Apr. 3, 1951 FEED-BACK CIRCUIT Theodore A. Rich and John E. Bigelow, Schenectady, N. Y., assignors to General Electric Company, a corporation of New York Application April 26, 1949, Serial No. 89,676

2 Claims.

Our invention relates to amplifying devices, and has as its principal object to provide a new and improved feedback arrangement, whereby a gradual reduction in amplifier gain may be efiected with increase in the frequency of an input signal impressed upon the amplifier, without accompanying excessive phaseshift between the input signal and the amplified output signal corresponding thereto, which may produce oscillatory conditions in a system, such as a closedcycle servo-system, wherein such an amplifier may be employed.

Our invention has found particular application in servo-systems of the type employing light beams to provide normal closed-cycle feedback. Such a light beam, when made responsive to the output signal of an amplifier, may be made to vary the impedances of photosensitive electron discharge devices connected in the input circuit of the amplifier. In the broadest aspect of our invention, an electrical feedback signal responsive to the output voltage of an amplifier, and supplementaryto a-light beam or other type of normal feedback which may be'employed, is impressed in a series relation upon the input circuit of the amplifier to modify the response characteristics thereof. Other objects and advantages of "our invention will become apparent from a-consideration of the following description when taken in connection with the figures of the accompanying drawing, wherein Fig. 1 is a schematic diagram of a preferred embodiment of our invention, including a feedback arrangement; Fig. 2 is a simplified equivalent circuit diagram of the arrangement of Fig. 1, but with feedback omitted; Fig. 3 is an equivalent circuit diagram of the arrangement of Fig. 2 with feedback added; and Fig. 4 is a graphical representation of certain operating characteristics of the arrangements of Figs. 2 and 3.

Referring now to Fig. 1, there is shown a closed-cycle servo-system arranged to provide an output voltage e0 substantially equal in magnitude to an unknown signal voltage en. The servo-system shown comprises, generally, an amplifier I having an input circuit 2 and an out.- put circuit 3, a detector 4, a light source 5, and an indicating element 6. Amplifier I includes an electron discharge device I having a cathode 8, an anode 9, and a control electrode III. The potential required by electron discharge device I may be provided by a source of potential such as a battery I I. The positive terminal I2 of battery I I is connected to anode 9 through a currentlimiting resistor I3, and a tap I4 of battery II, negative with respect to point I2, is connected to cathode 8.

Input circuit 2 includes a pair of photosensitive electron discharge devices I5 and I6, having anodes'I'l and I8 and cathodes I9 and 2G, respectively. Cathode I9 of electron discharge device I5 is connected to anode I8 of device I6, and the common point therebetween is connected to control electrode Ill of electron discharge device 1. Anode I! is connected through a current-limiting resistor 2| to a point 22 of battery II, which is negative with respect to point I2. Cathode 2B is connected through a feedback resistor 23, designated as R2, to a point 24 of battery II, which is negative with respect to point I 4.

It will be understood that thisinput circuit 2 is formed by photosensitive devices I5 and I 6, resistors 2| and 23, and the portion of battery II between points 22 and 24-, and that the potential of battery II tends to force a current through this circuit. The magnitude of such a current is dependent upon the impedances of photosensitive devices I5 and It, as determined by the amount of-light falling thereupon, as will be understood by those skilled in the art. Thus, the potential of control electrode III with respect to cathode 8 is dependent onthe relative amounts of light falling on photosensitive devices I5 and I6. As the potential of control electrode II! is varied withrespect to'cathode 8, the potential of anode 9 bein fixed with respect to cathode 8, the output voltage e0 across terminals 213 and 25 of amplifier I is likewise varied. The magnitude of voltage as may be indicated by indicator 5 which is connected across terminals 25 and 2t.

Detector 4 may be a galvanometer having a moving element 2'! biased to the center of its travel by a torsional spring 28, and having a mirror 29 affixed thereto. One terminal of detector 4 is connected to output terminal 25 of amplifier I, while the other terminal of detector 4 is connected to one terminal of unknown voltage e0, designated as point 30. The other terminal of voltage e0 is connected to the other output terminal 24 of amplifier I. Thus, in effect, detector 4 is made to sense the difference or error between voltages co and es which may be represented as (eo'eo). If these voltages are equal in magnitude the net voltages impressed on detector 4 is zero, and moving element 2'! is undeflected. If so is greater than e0 moving element 21 will be caused to move in one direction while if e0 is reater than e0 moving element 1 will be caused to move in the opposite direction.

In the system as thus far described, there is no feedback interconnection between the input circuit 2 of amplifier l and the voltage impressed on detector 4. Such an interconnection is provided by a light beam 3| originating at light source 5 and reflected by mirror 29, and arranged to fall on photosensitive devices 15 and I6. Light source 5 comprises a lamp 32 and a. source of potential such as a battery 33.

As previously noted, voltage e is dependent on the relative amounts of light falling on photo sensitive electron discharge devices and Hi, which in turn is dependent on the position of mirror 29 and moving element 21 of detector 4.

The function of the servo-system as herein represented and described, is to provide an output voltage co in the servo-system approximately equal to the signal voltage es impressed on the servo-system. However, voltages co and 60 are generally not exactly equal, but are unequal by the amount of error (0' ea) required by .detector 4 to deflect mirror '29 by an amount sufficient to cause amplifier l to provide the required value of voltage co to produce acondition of equilibrium. A servo-system of the type described is well known in the art and does not of itself constitute a part of our invention. ,It will be understood that the embodiment herein represented is included to assist in the understanding of our invention whichwill be presently described.

While no reference has been made to the frequency of signal voltage 60', it isassumed that the frequency thereof may vary over a considerable frequency range. Accordingly, voltage e0 likewise varies in frequency with e0, :but due to time lags in the various elements of the servo-system, as for example in the motion of moving element 29, voltages -80 and co may be considerably displaced in phase with respect to each other. Furthermore, as is will known, the gain ofrthe servo- .system, represented as 60/ (co-e0) ,decreases, .due

to various factors, as the frequency impressed on the-system is increased. As is well known, if the phase shift of voltages -60 and co reaches 180 before the system gain is reduced to unity, an oscillatory conditionmay be established in the servo-system which, in general, is highly undesirable,

To bring about gradual reduction in system gain without excessive phase shift, we provide a feedback arrangement wherein a feedback signal is connected in series with photosensitive'devices l5 and it. Such a signal is :provided by connecting the output voltage c0 .of' amplifier 1 across resistance 23 (R2) through a coupling capacitance 34, designated as C2. Additional feedback may be provided by impressing'voltage e0 across the combination of resistance 2-3 and photosenstive devices l5'and It, through a coupling capacitance 35, designated as C2. i-Sinoe photosenstive devices i5 and lBthaveeflectiVe'resistance and capacitance'Ri and Cl respectively, voltage 30 in effect is impressed across-a :total resistance .(R1+Rz) through capacitance C2. The effect offeedback "of the typeidescribed will:be explained more fully hereinafter in -simplified equivalent arrangements.

Referringnow'to'Fig. 2, thereis showniniblock form an amplifier 'I, having an amplifying constant-K, which may be considered as :an equivalent of-amplifier linIFi-g. '1. Amplifier I is'provid'ed with=an input circuit 2 andanoutputcircuit 3. .lnputicircuit -2 has;aminherentieffective capacitance C3 represented by capacitance 36.

The resistance and capacitance of devices associated with input circuit 2, such as photosensitive electron discharge devices 15 and IS in Fig. 1, are represented in Fig. 2 by a resistanc 31, designated as R1, connected in series with input circuit 2, and a capacitance 38, designated as C1, connected in parallel with input circuit 2 and also in parallel with capacitance 38. The input voltage impressed on input circuit 2 is represented as as and corresponds to the voltage 25 in Fig. 1 which is impressed across control electrode l0 and point 24 in the input circuit of amplifier I.

In Fig. 2, output circuit 3 is represented as providing output voltage e0. It will be noted that in the arrangement of Fig. 2, no feedback is provided between co and es. In other words, the arrangement shown represents only a simple amplifier provided with an input voltage es and having an output voltage e0.

Referr ing now to Fig. 3, an arrangement similar to Fig. 2 is shown, but with the addition'of a feedback circuit 39. Feedback circuit 39 is provided with a feedback constant selector 40 having a feedback constant L. Output voltage a; is impressed on selector 4!? and a voltage 60L is in turn impressed through a blocking capacitance 3 designated as C2, on a feedback resistor 23, designated as R2, connected in series with input circuit 2 between capacitance C1 and C3. The arrangement of Fig. 3 is equivalent to the arrangement of Fig. 1, except that it does not represent a closed-cycle servo-system, but represents only amplifier I, input circuit 2, and output circuit 3, and a feedback arrangement of the .type shown in Fig. 1 interposed between the output andinput circuits.

In Fig. 4, there are shown certain characteristics of the arrangements of Figs. 2 and 3. In particular, amplifier gain, represented by (co/es), particular, amplifier gain, represented by (60/6 is plotted against the frequency f of input voltage es. It is assumed that the typical values ,given in :thefollowing table have been assigned to the .constants of Figs. 2- and ;3.

In Fig. 4, curve 4'! represents the approximate characteristic of amplifier l without the use of feedback, as represented in Fig.2, while curve '42 represents the approximate characteristic of amplifier 4- with the addition of feedback, as representedin' Fig. 3. It will be noted by reference to curve-G l, that with increasing frequency f of input voltage es, the gain of-arnplifier I without feedback tends to remain constant over a considerable range of frequency and steadily decreases beyond a-critical value of frequency, in-

dicated as point, 43 on curve 4|. If feedback is included, however, the gain of amplifier l is initially constant over a' fairly considerable range but has an initial decrease beginning at a substantially lowervalue of frequency, indicated as point A l-on curve 42, than is the case without feedback. The-initial' decrease of gain continues until'point '45 on curve His-reached, after which the :gain once again becomes constant over a fairly considerable-range and-ultimately-decreases atzpoint 45 in a manner similar to .the decrease without feedback. It will be understood that for convenience curves '41- and are approximate representationsofthe characteristics of amplifier 1*.andzthat awhile points 43-46 .inclusivea conveniently represent intersections of the relatively straight portions of curves 4| and 42, in reality the transitions between such portions are gradual and not definite intersections as indicated in Fig. 1.

The shape of curve 42 may be explained as follows: In the region between points 45 and 4f, the negative feedback operates to reduce the amplification of the system, and therefore curve 42 lies substantially below curve 4|. At lower frequencies, the amount of feedback is reduced by the increased impedance at low frequencies of capacitor 34 relative to the impedance of resistor 23. At frequencies below point 44, the impedance of capacitor 34 is so great relative to that of resistor 23 that the feedback is negligible, andthe full amplifier gain is obtained. Thus the negative feedback is inoperative to reduce the amplifier gain at very low frequencies, and becomes effective to reduce the gain at higher frequencies, thus preventing oscillations. The frequency range at which the feedback becomes effective depends upon the time constant of capacitor 34 and resistor 23, and can be made to have desired values by proper choice of the capacitance and resistance values.

We have found a frequency response characteristic of the type represented by curve 42 of Fig; 4 to be particularly useful when employed in coniunction with servo-systems of the type embodied in Fig.1. We have found that by making use of the f edback arrangement of our inven tion to provide a reduction of amplifier and servosystem gain with increasing frequency of a signal impressed upon the servo-system, without introduction of accompanying excessive phase shift therein, the prevention of undesirable oscillatory conditions in the servo-system is materially facilitated. Accordingly, in a stable servo-system, use of the feedback arrangement of our invention permits higher gains to be employed in the low frequency range than would otherwise be the case, without impairing stability in the high frequency range and with accompanying improved performance characteristics.

While we have shown a preferred embodiment and app ication of our invention. inc uding a particular system of connections, it will be obvious to those skilled in the art that our invention may well take other forms and may be employed in other applications. We, therefore, aim in the appended claims to cover all such modifications as fall within the true spirit and scope of our invention.

, What we claim as new and desire to'secure by Letters Patent of the United States is:

1. In a closed-cycle photoelectric servo system, the combination of a direct coupled amplifier having input and output terminals, an electron discharge device having an anode and a photosensitive cathode, a resistor connected in series with said discharge device across said input terminals, and a capacitor connected in series with said resistor across said output terminals, whereby negative feedback voltage is applied across said resistor at frequencies above a value determined by the time constant of said resistor and capacitor, thereby preventing oscillation of the system.

2. A closed-cycle photoelectric servo system, comprising first and second electron discharge devices each having an anode and a photosensitive cathode, the cathode of said first device being connected to the anode of said second device, a resistor connected to the cathode of said second device, means to apply direct voltage across said firstand second devices and said resistor in series, a third electron discharge device having an anode, a control electrode, and a cathode, said' control electrode being directly connected'to the circuit junction between said first and second devices, a mirror galvanometer connected to the anode of said third device, a light source, said galvanometer and light source being arranged to illuminate the photosensitive cathods of said first and second devices by different relative amounts upon rotation of the galvanometer mirror, and a capacitor connected between the anode of said third device and the circuit junction between said second device and said resistor, whereby negative feedback voltage is applied across said resistor at frequencies above a value determined by the time constant of said resistor and capacitor, thereby preventing osci lation of the system.

THEODORE A. RICH. JOHN E. BIGELOW.

REFERENCES CITED lhe following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,809,676 Culver June 9, 1931 2,082,627 Haugh June 1, 1937 2,469,852 Strutt et al May 10, 1949 2,506,384 Rich May 2, 1950 

